This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to removing phase errors in MRI data.
Magnetic Resonance Imaging (MRI) is based on careful manipulation of phase information in the NMR signal. Phase errors are therefore a significant problem in MRI. The correction of static phase errors, as may arise from magnetic field inhomogeneties has been extensively studied. However, the more complicated situation of time-varying phase errors has been incompletely treated. This set of problems involves phase errors which vary from readout to readout.
Imaging situations that require the correction of time-varying phase offsets include diffusion-weighted imaging (DWI), multi-coil imaging with time-varying coil sensitivity patterns and moving-table imaging with time-varying field maps.
As an example, consider diffusion-weighted imaging. When DWI data is gathered in a multi-shot acquisition, bulk motion introduces varying phase shifts following different excitations. These phase errors are problematic for multi-shot acquisitions because each readout experiences a different image-space phase corruption, causing phase interference when readouts are combined to form a high-resolution image. Navigated techniques correct phase errors based on a low resolution phase reference (or navigator) that is acquired along with each high-resolution data frame.
Navigated DWI methods typically assume rigid-body motion, the effects of which have been well characterized. However, a significant component of brain motion is non-rigid. During systole, the brain deforms about the ventricles and displaces inferiorly through the foramen magnum as though being pulled by the spinal cord. The more superior and lateral regions of the brain are nearly motionless, whereas the more inferior and medial regions tend to experience large displacements. Even during the relative quiescence of diastole, motion induced by the cardiac cycle follows this nonlinear spatial pattern as the brain slowly returns to its resting position.
In the presence of diffusion-weighted gradients, these deformations can cause phase corruptions that vary nonlinearly across the object magnetization. The effect of brain motion on the DWI signal is shown in FIG. 1, which is an illustration of the effects of motion on diffusion-weighted magnetization. The magnitude (top) and phase (bottom) of a series of low-resolution axial single-shot DWI images (2D navigators) are shown over a single cardiac cycle. The magnitude and phase of a representative voxel are plotted in the middle over four cardiac cycles. The spatial variation in the image magnitude and phase is nonlinear and strongly correlated to the cardiac cycle. The signal phase and amplitude are correlated with the cardiac cycle and exhibit nonlinear spatial variation. The loss in signal amplitude during peak systole is due to intravoxel dephasing caused y intense motion. The highly focal nature of this dephasing highlights the spatial nonlinearity of phase corruptions.
These phase errors are problematic for multi-shot acquisitions because each readout experiences a different image-space phase corruption (φj(r) for the jth readout). This causes phase interference when readouts are combined to form a high-resolution image. Navigated techniques correct phase errors with low-resolution navigator data that is acquired along with each high-resolution data frame. Provided the navigator data (a) samples the same phase error φj(r) as the high-resolution data, and (b) covers k-space sufficiently to resolve φj(r), the navigator can be used to correct phase errors in the high-resolution data before the interleaves are combined. Based on the assumption of rigid-body motion, standard navigator methods correct for 0th and 1st order phase terms. In the presence of the non-linear errors described above, a rigid-body correction is a first-order approximation to the desired high-order correction. Given that a 2D navigator contains high-order phase information, image reconstruction should be improved by correcting for higher-order phase terms.